Fired heater arrangement for film temperature control



H. L. BEGGS July 5, 1966 3 Sheets-Sheet l Filed April 28, 1964 nu. m M 2 1I f EL, n 0 2 hwg P n O /nl. (9 O m o w m Moooooooooo^mM%(wOO x M 2 1.6,., o mm o 6 il@ (5 O 0J o A M NJ 2 d n ooo o ooooooOOO 3 m il v. O n 3 w 6. Aw 9 w 9 ,S4 11 l l o m" Moooo oooooOOO m, M Il: Muy/5 ,z f/H O w \l\ n" o 6. Q/[Mund 9 O n" o 2 L no w d 4M/vojo; oooooooo ooo O, A n o ^Il. m O d w ATTORNEY July 5, 1966 H. L. BEGGS 3,259,110

FIRED HEATER ARRANGEMENT FOR FILM TEMPERATURE CONTROL Filed April 28, 1964 5 Sheets-Sheet 2 INVENTOR HAPOLO -5.5665

ATTORNEY H. 1 BEGGs 3,259,110

FIRED HEATER ARRANGEMENT FOR FILM TEMPERATURE CONTROL 3 Sheets-Sheet 3 He. E

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OOOGOO INVENTOR ,fm/@04a 4. 5566s ATTORNEY July 5, 1966 Filed April 2e, 1964 United States Patent O l" 3,259,110 FIRED HEATER ARRANGEMENT FOR FILM TEMPERATURE CONTROL Harold L. Beggs, St. Davids, Pa., assignor to Alcorn Combustion Company, New York, N.Y., a corporation of Delaware Filed Apr. 28, 1964, Ser. No. 363,182

2 Claims. (Cl. 122-356) This disclosure relates to fired heaters used in the petroleum refining industry. The invention contemplates a novel arrangement for film temperature control.

In fluid heaters such as those employed in vacuum service, it is desired to elevate the bulk temperature of the uid -to be heated to the level required for vaporization without subjecting the fluid to thermal cracking. Thermal cracking of the uid wastes ingredients; but worse than this, cracking usually deposits coke in the tube coils. By impeding heat transfer to the fluid, coke deposits in the tube coils hasten burning through of tube walls. In this regard hot spots on the interiors of tube walls raise point film temperatures and increase the -tendency toward thermal cracking.

l One approach toward mitigating film temperatures has been to double fire the tube coils. Radiant tube coils are generally fired from either one side (single fired) or from opposite sides (double fired). Double firing evens the heat input about the peripheries of the tubes. So with double firing, for a given heat input, the maximum point film temperature is less than that for single firing. But double fir-ing alone is not generally enough.

The thermal cracking problem is aggravated by large tube sizes. In vacuum installations, with an increase in vapor pressure, larger tube sizes sometimes become necessary along the path of the uid through the coils to keep fluid pressure drops within reasonable limits. The enlarged tubes avoid suppressing vaporization thereby avoiding super heating the process fluid in its liquid state, but these larger tube sizes involve a reduction of the mass velocity through these tubes impairing the ability of the process stream to cool the tube metal. With reduced cooling the point temperature of process fluid at the wall of these enlarged tubes elevates and the tendency toward thermal cracking becomes most critical. Towardthe outlets of the tube coils where tube sizes are largest, the thermal cracking problem is most acute.

The present teaching offers a heater design which avoids thermal cracking in a novel manner. A vacuum heater is provided with a radiant tube coil having three zones which communicate in flow series along the length of the flow path of fluid through the coil. In the first zone, single firing of the coil is employed because process fluid temperatures in this zone are too low for cracking to be a problem. Then in the second zone as higher temperatures are reached double firing of the coil is resorted to. Later along the path of the uid when relatively large tube diameters are required, and the thermal cracking problem becomes most severe, shielding walls are arranged to reduce the heat input rate to this zone of the coil. Checkered shielding walls can be used where a portion of the heat from a main burner is to be introduced to the large diameter critical tubes.

For greater flexibility of operation the shielding walls can be made solid and trim burners may be installed on the sides of the shielding Walls adjacent the critical tubes.

These and other features will appear more fully from the accompanying drawings wherein:

FIGURE I is an end view in section which depicts a tired heater including the present advance. Solid shielding walls and trim burners are used in this embodiment. 7

FIGURE I is taken along line I-I of FIGURE II.

y 3,259,110 Patented July 5, 1966 FIGURE II is a side view taken in part along line II--II pf FIGURE I.

FIGURE III is an end view in section which shows the checkered shielding wall embodiment of this invention.

FIGURE IV is an enlarged detail of a checkered shielding wall.

Basically, these designs call upon known design principles and market-available components. Setting 1 (with enclosure walls 2, roof arch 3 and floor 4) defines radiant section 6 therebetween. Steel lframe 7 transmits loads to foundations `8.

In the embodiment shown in FIGURES I and II, rows of main burners 9 as well as rows 11 of smaller trim burners penetrate setting 1 via floor 4 and communicate with radiant section 6 to introduce fuel an-d air therein. Burners 9 and 11 are operatively associated with setting 1 so that main fia-me bursts 12 and trim flame bursts 13 are formed and combustion gases are generated. The combustion gases exit radiant section 6 via convection section 14 and stack 16.

Before a process stream to be heated has reached high temperatures in radiant section 6, there is little danger of thermal cracking. So at low temperatures single firing of the coils can be employed. Process fluid (supplied from a source not shown) after being preheated in convection coils 17 is coursed in ow series through one of the banks of roof tubes 18 disposed adjacent roof arch 3 for single firing therein.

As the temperature of the process stream increases, double firing of the coils is generally necessary so that maximum film temperature can be kept within acceptable limits. Toward this objective main burners 9 are arranged so that main flame bursts 12 are spaced from each other. Upright coils of tubes 19 are disposed between adjacent main flame bursts 12` for double firing. Radiant tubes 19 are disposed horizontally in each coil to define generally horizontal serpentine configurations. From roof tubes 18 the process fluid is conducted in flow series to pass downward through one of process fluid exits by way of downstream ends 21 of the upright coils.

As the process fluid courses downward .through the upright horizontal coils, it is usually necessary to increase tube diameters so that vapor suppression is avoided and so that pressure drop through the radiant tubes is kept within reasonable limits. Downstream tubes, such as 22 have diameters sufficiently large that film temperature control becomes extremely difficult. These are the critical tubes. To protect downstream tubes 22, shield means shown as shielding walls 23 are provided .to limit heat transfer trom main flame bursts 12 to tubes 22.

Heat input to downstream tubes 22 must be kept low enough to limit film temperatures. Secondary means are provided -to add lower heat input rates Vto downstream tubes 22 than are available from exposure of these 4tubes to .main flame bursts 12. In the embodiment of FIG- URES I and II the secondary means comprise small trim burners 11 disposed on the sides of shielding walls 23 adjacent downstream tubes 22. Adjustment of trim burners 11 offers sensitive control of film temperature in downstream tubes 22.

In the embodiment shown in FIGURES III and IV, no trim burners are used. Instead, shielding walls 23 define checker openings 24.

It will be apparent to those skilled in furnace design that wide deviations in the detail of this disclosure can be made without departing from the main theme of invention as set forth in the claims.

What is claimed is:

1. A vacuum heater comprising a setting which has a door and defines a radiant chamber,

a plurality of main burners penetrating the setting and communicating with the radiant chamber to introduce fuel and air therein and arranged so that two main flame bursts are formed and combustion gases are generated,

means for exhausting the combustion gases from the radiant chamber,

at least one tube coil disposed in the radiant chamber to dene a generally horizontal serpentine contiguration,

the tube coil communicating in llow series with a source of liquid hydrocarbon so that the liquid hydrocarbon courses generally downward therethrough,

the main burners arranged so that the main llame bursts are spaced from each other,

the tube coil disposed between the main flame bursts so that the coil is double tired,

the coil including at least one critical tube having a substantially enlarged diameter and path of liquid hydrocarbon ilow, shield means for limiting heat transfer from the main ame bursts to the critical tube,

the main burners penetrating the setting via the oor,

`the shield means including a Ipair of shielding walls each positioned between one of the main llame bursts and the tube coil and each projecting upward from the floor approximately as high as the top of the critical tube,

trim burners disposed on each side of the critical tube and between the shielding walls,

means for adjusting the heat output of the trim burners.

2. A Vacuum heater comprising a setting having a roof arch as well as a floor and defining a radiant chamber therein,

main burners penetrating the setting via the floor and communicating with the radiant chamber to introduce fuel and air therein and arranged so that two main llame bursts are formed and combustion gases are generated,

means for exhausting the combustion gases from the radiant chamber,

at least one roof tube disposed adjacent the roof arch and exposed to radiation from one of the main ame bursts for single ring,

means communicating the roof tube in flow series with a source of liquid hydrocarbon so that the liquid hydrocarbon courses therethrough,

at least one bank of radiant tubes in the radiant chamber and connected in flow series downstream of the roof tubes to accept the liquid hydrocarbon therefrom,

means remote from the roof, to exhaust the liquid hydrocarbon from the radiant tubes,

the main burners arranged so that the main arne bursts are spaced horizontally from each other,

the radiant tubes disposed between the main llame bursts so that they are double fired,

the radiant tubes disposed horizontally to define a generally horizontal serpentine configuration arranged `for the liquid hydrocarbon to course downward therethrough,

the radiant tubes including at least one critical tube situated downstream in the path of ow through the radiant tubes,

shield means for limiting the heat transfer from the llame bursts to the critical tube,

trim burners on each side of the critical tube and between the `shielding walls,

means for adjusting the heat output of the trim burners.

References Cited by the Examiner UNITED STATES PATENTS 2,017,243 10/ 1935 Gibson 122-356 2,029,292 2/ 1936 Alther 122-356 2,557,569 6/1951 Schutt 196-110 2,667,449 l/ 1954 Mekler 122-346 X 2,856,902 `10/-1958 Throckmorton et al. 122-356 KENNETH W. SPRAGUE, Primary Examiner. 

1. A VACUUM HEATER COMPRISING A SETTING WHICH HAS A FLOOR AND DEFINES A RADIANT CHAMBER, A PLURALITY OF MAIN BURNERS PENETRATING THE SETTING AND COMMUNICATING WITH THE RADIANT CHAMBER TO INTRODUCE FUEL AND AIR THEREIN AND ARRANGED SO THAT TWO MAIN FLAME BURSTS ARE FORMED AND COMBUSTION GASES ARE GENERATED, MEANS FOR EXHAUSTING THE COMBUSTION GASES FROM THE RADIANT CHAMBER, AT LEAST ONE TUBE COIL DISPOSED IN THE RADIANT CHAMBER TO DEFINE A GENERALLY HORIZONTAL SERPENTINE CONFIGURATION, THE TUBE COIL COMMUNICATING IN FLOW SERIES WITH A SOURCE OF LIQUID HYDROCARBON SO THAT THE LIQUID HYDROCARBON COURSES GENERALLY DOWNWARD THERETHROUGH, THE MAIN BURNER ARRANGED SO THAT THE MAIN FLAME BURSTS ARE SPACED FROM EACH OTHER, THE TUBE COIL DISPOSED BETWEEN THE MAIN FLAME BURSTS SO THAT THE COIL IS DOUBLE FIRED, THE COIL INCLUDING AT LEAST ONE CRITICAL TUBE HAVING A SUBSTANTIALLY ENLARGED DIAMETER AND PATH OF LIQUID HYDROCARBON FLOW, SHIELD MEANS FOR LIMITING HEAT TRANSFER FROM THE MAIN FLAME BURSTS TEO THE CRITICAL TUBE, THE MAIN BURNERS PENETRATING THE SETTING VIA THE FLOOR, THE SHIELD MEANS INCLUDING A PAIR OF SHIELDING WALLS EACH POSITIONED BETWEEN ONE OF SAID MAIN FLAME BURSTS AND THE TUBE COIL AND EACH PROJECTING UPWARD FROM THE FLOOR APPROXIMATELY AS HIGH AS THE TOP OF THE CRITICAL TUBE, TRIM BURNERS DISPOSED ON EACH SIDE OF THE CRITICAL TUBE AND BETWEEN THE SHIELDING WALLS, MEANS FOR ADJUSTING THE HEAT OUTPUT OF THE TRIM BURNERS. 